gitlog-to-changelog: do not clump multi-paragraph entries

gitlog-to-changelog: do not clump multi-paragraph entries

Identical header lines (date,name,email+coauthors) are suppressed,
thus putting all entries with those same characteristics under
a single header.  However, when a log entry consists of two or
more paragraphs, it may not be clear where it starts and ends.
This change makes it so that such an entry is always separated
from others by a header line, even when that header would
otherwise be suppressed.
* build-aux/gitlog-to-changelog: Implement the above.
Inspired by a related request from Stefano Lattarini in
http://thread.gmane.org/gmane.comp.lib.gnulib.bugs/29456